1. Field of the Invention
The present invention relates to improvements in an optical scanning device for scanning a subject medium.
2. Description of the Related Art
In recent years, with development of digital techniques and light sources, numerous high performance printing equipment and the like have been developed, wherein a light beam modulated according to image signals scans a subject matter, such as photographic pictures and printed matter (which is hereafter referred to as a scanning subject medium), that is sensitive to light to record an image on a paper. In such a printing equipment, the utilization is made of an optical scanning device to scan a scanning subject medium. This optical scanning device is, for typical example, configured such that a laser beam from a laser diode is repeatedly reflected and deflected by a rotary polygon mirror and further directed as a scanning beam to a subject medium through an f-xcex8 lens. Because deflection of the scanning beam by the polygon mirror causes defectiveness in the evenness of velocity of straight line motion of the scanning beam along a canning line on the scanning subject medium relative to a temporal change in rotational angle xcex8 of the polygon mirror, the f-xcex8 lens is employed in order for the optical scanning device to avoid the defectiveness in the evenness of velocity of straight line motion of the scanning beam along a scanning line and keep the velocity of straight line motion of the scanning beam.
In this type of optical scanning device, the polygon mirror that rotates at a speed sufficiently high to cause a current of air therearound stirs up dust and, in consequence, possibly gathers fine dust on its reflective surfaces. If the reflective surface of the polygon mirror gets dust once, the reflective surface causes a scanning beam to scatter or absorbs a scanning beam with the dust, which leads to a basic problem that the scanning beam looses desired accuracy of light intensity and, in consequence, encounters aggravation of precise scanning.
Although this problem is thought to be eliminated by enclosing the polygon mirror in a dust proof chamber, nevertheless, the polygon mirror is hard to be placed alone in the dust proof chamber, and it is necessary to place an electric motor for driving the polygon mirror and its associated parts such as a motor drive circuit and the like. The dust proof chamber encounter a significant rise in ambient temperature due to heat generated rotation of the electric motor and operation of the motor drive circuit, which is always undesirable in light of operational circumstances for the polygon mirror that is a precise optical element that is sensitive to a change in temperature. For example, the polygon mirror causes surface distortion due to a sharp rise in ambient temperature with an adverse effect of aggravation of scanning accuracy.
It is an object of the present invention to provide an optical scanning device which is accurate in scanning operation.
The foregoing object of the present invention is accomplished by providing an optical scanning device for scanning a subject medium with a light beam that is reflected by a motor driven polygon mirror, which has a dust proof chamber formed in a generally rectangular box-shaped housing for receiving an electric motor and a polygon mirror driven by the electric motor therein and heat releasing means such as a cooling fin arrangement having a plurality of cooling fins for releasing heat within the dust proof chamber.
The interior of the dust proof chamber is preferably connected to the heat releasing means through heat conduction means which may be a heat pipe or a base board with which the cooling fins are integrally formed and which forms at least one of walls of the said dust proof chamber.
An f-xcex8 lens system, which is necessary to implement accurate scanning and usually comprises at least two lens elements, may be installed such that one of the lens elements is directly secured to one of walls of the dust proof chamber excepting the walls that are formed by the base board.
Since the dust proof chamber, which prevents the polygon mirror from getting dust, is thermally connected to the heat releasing means such as a cooling fin arrangement, the interior of the dust proof chamber is kept from a rise in temperature above a certain level. Accordingly, the optical scanning device is prevented from causing inaccurate scanning due to dust adhesion on the polygon mirror and surface distortion of the polygon mirror that is caused by a rise in ambient temperature
Many types of heat conduction means, which is well known in various forms and may take any known form, transmits heat in the dust proof chamber efficiently to the heat releasing means, so as to cool effectively the interior of the dust proof chamber.
When enclosing a polygon mirror in the dust proof chamber, it is essential to form an outlet opening for providing an optical path of the scanning beam in one of walls of the duct proof chamber. Although an inlet opening through which the scanning beam enters into the dust proof chamber can be small because the scanning beam does not oscillate, the scanning beam outlet opening has to be wide sufficiently enough to permit oscillation of the scanning beam. Hermetically fitting an f-xcex8 lens or a component lens element of an f-xcex8 lens system in the laser beam outlet opening eliminates a transparent protective glass for closing the laser beam outlet opening, which is always desirable for reducing the number of optical parts and contributory to miniaturization of the optica scanning device.